1. Technical Field
The present invention relates to a contact nozzle for electric arc welding and also a spring element for mounting in such a contact nozzle and also a welding torch with such a contact nozzle.
2. Background
In arc welding with mechanized feed of continuous welding wire, where the welding current flows through the welding wire, use is made of a contact nozzle, which can be mounted in a welding torch, with two main functions. Firstly, the contact nozzle is to guide mechanically the delivery of the welding wire from the welding torch so that the arc which is established during welding between the welding wire end and the welding workpiece has a repeatable and constant point of impact on the workpiece under otherwise unchanged conditions of the workpiece relative to the welding torch. Secondly, the contact nozzle is to bring about reliable and repetitive welding current transfer from the contact nozzle to the welding wire.
A contact nozzle of the kind mentioned above is normally made of pure copper or a copper alloy, or alternatively by powder metallurgy where copper constitutes an essential part. It is in the main designed as a tube with a through-hole for passage of the welding wire. One end is designed for connection to the welding torch, in most cases by means of a thread 8. The other end is designed so as to minimize the amount of welding spatter which becomes attached to the contact nozzle, for example by means of a slightly conical shape with a rounded end surface. The contact nozzle is often provided with a wrench grip so it is easier to take hold of and to detach from the welding torch.
Contact nozzles of the above kind are a consumable part in welding. In the case of automated welding, it often happens that the nozzle is exchanged, as a preventive measure, a number of times a day in order to avoid welding defects occurring during production. This involves a considerable cost in terms of both nozzles themselves and lost production time. The wear of the nozzles manifests itself in, for example, the output hole being enlarged and thus impairing the ability of the nozzle to guide the wire. This wear is caused partly by pure and simple mechanical wear as the wire passes through the nozzle but also by electric erosion in connection with spark formation when current transfer from contact nozzle to welding wire takes place. Another sign of a worn nozzle may be that the contact transfer of the welding current begins to a greater extent to migrate between different points in the nozzle, which results in varying welding characteristics by virtue of the current flowing through a shorter or longer part of the wire and thus the voltage available for the arc varying. In order to obtain a good and repeatable position of the contact transfer in the nozzle, it is of great importance that the radius of curvature of the welding wire, brought about by, for example, winding on a reel, is kept within predefined limits.
Various methods of increasing the life of the contact nozzles have been tried with varying success and at varying cost. One known method is to provide the contact nozzle with a lining made of harder material than the rest of the nozzle, preferably at the outlet end for the wire. This prevents enlargement of the outlet hole as a consequence of mechanical wear. If the lining is insulating as well, no electric erosion arises either, as a result of which the nozzle retains its ability to guide the welding wire for a considerably longer time. A disadvantage is that the contact transfer has a distinct tendency towards becoming more migratory along the length of the nozzle with the consequence that the welding process becomes more unstable.
Another method has been to stabilize the contact transfer with the aid of a spring element applied through a hole drilled radially from the side in the contact nozzle. This can provide a good and positionally stable current transfer. However, a disadvantage is the cost for both extra machining of the contact tube and mounting of the spring element.